(1) Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical transmission system having a function for controlling the optical signal power of each wavelength included in WDM light, and more specifically, relates to a technique for stabilizing control of the optical signal power for a WDM optical transmission system in which an optical add-drop station and an optical cross-connect station are arranged on an optical transmission line.
(2) Related Art
In the basic configuration of a conventional WDM optical transmission system, as shown for example in FIG. 12, a point-to-point configuration is generally used, in which optical signals of wavelengths λ1 to λ3 from transmitters (E/O) 101 are multiplexed by a wavelength multiplexing (MUX) station 102 to transmit the WDM light to an optical transmission line 103, and the WDM light is repeatedly transmitted, while being amplified by a repeating and amplifying station (REP) 104 arranged on the optical transmission line 103. The WDM light is then demultiplexed into optical signals of respective wavelengths by a wavelength demultiplexing (DEMUX) station 105, and received by optical receivers (O/E) 106 corresponding to each wavelength.
In this conventional configuration, variable optical attenuators (VOA) 102A are provided, which perform level adjustment for each wavelength, so that the wavelength optical powers of the multiplexed WDM light become uniform with respect to the wavelength multiplexing station 102. At the time of increasing or decreasing the number of wavelengths, the attenuation by the respective variable optical attenuators 102A is adjusted so as to control the power of optical signals of the respective wavelengths. A multiplexer 102B in the wavelength multiplexing station 102 is for multiplexing the optical signals output from the respective variable optical attenuators 102A to generate the WDM light. An optical amplifier 102C is for amplifying the WDM light output from the multiplexer 102B to a necessary level. A level adjuster 102D is for controlling the respective variable optical attenuators 102A by monitoring the optical power of the respective wavelengths in the WDM light output from the optical amplifier 102C.
For the repeating and amplifying station (REP) 104, there is a known technique (for example, see Japanese Unexamined Patent Publication No. 2000-244411), in which the total power of the WDM light output to the optical transmission line 103 is made uniform by applying automatic level control (ALC), and when the number of wavelengths in the WDM light fluctuates, automatic gain control (AGC) is performed to reduce the change in the gain-wavelength characteristic, thereby preventing deterioration in the transmission quality.
Recently, a WDM optical transmission system having an optical add-drop function for adding or dropping a part of the optical signals included in the WDM light on the optical transmission line directly in the state of light, without converting it to an electric signal, and an optical cross-connect function for changing over the optical path of the optical signal in each wavelength by wavelength conversion or the like has been developed. The configuration shown for example in FIG. 13 is one example of the conventional WDM optical transmission system in which adding or dropping of the optical signal of a wavelength λ3 is performed by an optical add-drop (OADM) station 107 provided on the optical transmission line 103. In the case of the system configuration having a path changing station such as the optical add-drop station or the optical cross-connect station, it is necessary to equalize the power of optical signals of the respective wavelengths in the path changing stations, as in the point-to-point configuration. Therefore, it is necessary to control the optical power by using a variable optical attenuator (VOA) or the like corresponding to the respective wavelengths.
In the conventional WDM optical transmission system, when the point-to-point configuration as shown in FIG. 12 is used, the control of the power of the optical signals of the respective wavelengths is performed only in the wavelength multiplexing station 102, and the respective optical signals input to the wavelength multiplexing station 102 are light output from the respective optical transmitters 101, with the wavelength and the power adjusted to an appropriate level. On the other hand, in the case of the system configuration including the path changing station such as the optical add-drop station 107, as shown in FIG. 13, not only the wavelength multiplexing station 102 but also the optical add-drop station 107 control the optical signal power. As a result, the optical power is controlled in a plurality of places, until the optical signals output from the respective optical transmitters 101 are received by the respective optical receivers 106 via the optical transmission line 103.
When with respect to the conventional WDM optical transmission system including such a path changing station, the power of optical signals of the respective wavelengths is controlled by a method similar to the point-to-point configuration, there are problems described below.
In the configuration including the optical add-drop station 107 on the optical path from the wavelength multiplexing station 102 to the wavelength demultiplexing station 105 as shown in FIG. 13, when an increase in the number of wavelengths is performed the number of signals of different wavelengths is increased in the wavelength multiplexing station 102 on the upstream side, the optical signals of the increased wavelengths are input to the optical add-drop station 107 on the downstream side, and the optical add-drop station 107 starts addition control, triggered by the recovery of the input cut-off state of the relevant wavelength. Since the level adjuster in the optical add-drop station 107 has the same function as that of the level adjuster in the wavelength multiplexing station 102, if the power of the input optical signal is stable, the feedback control can be converged normally. However, at the time of increasing the number of wavelengths as mentioned above, the power of the input optical signal fluctuates dynamically. Therefore it becomes very difficult to adjust the optical signal power of the increased wavelength to an appropriate level by the optical add-drop station 107. As a result, increase in the number of wavelengths may not be completed normally, or retry of control has to be repeated, thereby taking a long time for activation (first problem).
FIGS. 14 and 15 show one specific example of the state at the time of increasing the number of wavelengths. For simplifying the explanation, as shown in FIG. 14, a configuration in which the repeating and amplifying station is omitted is assumed, wherein an output end of the wavelength multiplexing station 102 is designated as point A, an input end of the variable optical attenuator in the optical add-drop station 107 is designated as point B, and an output end of the optical add-drop station 107 is designated as point C. In FIG. 15, fluctuations in the optical signal power of the signal of the increased wavelength at the respective points are shown.
As in the example of FIG. 15, when the increase in wavelengths is started at time T0, the optical signal power of the increased wavelength at point A increases. At the time of starting to increase the number of wavelengths, the wavelength multiplexing station 102 controls the variable optical attenuators so that the optical signal power becomes a desired level. Moreover, the optical add-drop station 107 starts the control of the optical signal power of the increased wavelength, triggered when the optical signal power of the increased wavelength, which has been in the input cut-off state, exceeds a predetermined threshold (input cut-off recovery threshold) PTH at time T1. At this time, the optical signal power of the increased wavelength input to the variable optical attenuator in the optical add-drop station 107 dynamically fluctuates from time T0 to time T2, during which time the increase in the number of wavelengths is controlled by the wavelength multiplexing station 102 on the upstream side. Therefore, from time T1 to time T2, the control of the optical signal power of the increased wavelength by the wavelength multiplexing station 102 and the control of the optical signal power of the increased wavelength by the optical add-drop station 107 are performed at the same time, and the control state by the optical add-drop station 107 on the downstream side becomes unstable due to the influence of fluctuations in the optical signal power caused by the parallel control. As a result, the optical signal power of the increased wavelength at point C is not converged to the desired level, until a certain period of time has passed after completion of control on the upstream side to reach time T3. Particularly, when the optical add-drop stations (or the optical cross-connect stations) are connected in multiple stages, the influence of such a state becomes noticeable, thereby causing a problem.
Moreover, in addition to the first problem, the optical amplifiers provided in the wavelength multiplexing station and the repeating and amplifying station amplify and output signal components in the WDM light, and at the same time, output the spontaneously emitted light (ASE) as noise components. The noise light generated in the optical amplifiers remains in the output light from the wavelength multiplexing station 102 (see point A in FIG. 16), even when the optical signal is not output from the optical transmitter 101 for the wavelength λ1 (a decrease of the signal of the wavelength λ1) as shown in FIG. 16. Hence, the optical power of the wavelength λ1 input to the optical add-drop station 107 on the downstream side does not become zero (see point B in FIG. 16). Therefore, even when the wavelength λ1 is decreased in the transmission section on the upstream side, the optical add-drop station 107 may not be able to detect the input cut-off of the signal components, due to the ASE corresponding to the wavelength λ1. In this case, unnecessary light of only the ASE without including the signal component, is output from the optical add-drop station 107 for the wavelength λ1 (see point C in FIG. 16), and is further transmitted to the downstream side, thereby causing deterioration in the transmission characteristics of the WDM light (second problem).
Furthermore, in order to ensure high reliability, in the communication system, it is generally important that maintenance personnel can determine a problematic point in a short period of time at the time of failure. However, in the conventional system configuration including the path changing station such as the optical add-drop station, since the path changeover of the optical signals of the respective wavelengths included in the WDM light is carried out at a plurality of places, a failure on the upstream side affects the downstream side. Therefore, an abnormality at one place affects a plurality of places, thereby making it difficult to determine the fundamental problematic point in a short period of time (third problem).
In addition, other than the case in which the optical signals of the respective wavelengths included in the WDM light are increased or decreased, for example, when the optical signal power of the signals of the respective wavelengths changes due to fluctuations in the loss of the optical fibers, a similar situation to that of the first problem may occur. In other words, in the conventional configuration shown in FIG. 13, when the loss in the optical fibers fluctuates, the optical signal power level also changes. Therefore, the level adjusters in the wavelength multiplexing station 102 and the optical add-drop station 107 control the optical signal power, to perform the control for the whole system so that the level fluctuations can be absorbed (compensated). At this time, if the control of the optical signal power in the respective stations is performed at the same time, a change in the optical signal power due to the control on the upstream side affects the control on the downstream side, thereby taking a long time for the optical signal power control. Moreover, according to circumstances, oscillation may occur to make it difficult to perform normal control (fourth problem).